Fix ICE on const evaluation of const method

Author: estebank

src/librustc/ty/sty.rs

@@ -1007,6 +1007,16 @@ impl<'tcx> FnSig<'tcx> {
     pub fn output(&self) -> Ty<'tcx> {
         self.inputs_and_output[self.inputs_and_output.len() - 1]
     }
+
+    // Create a minimal `FnSig` to be used when encountering a `TyKind::Error` in a fallible method
+    fn fake() -> FnSig<'tcx> {
+        FnSig {
+            inputs_and_output: List::empty(),
+            c_variadic: false,
+            unsafety: hir::Unsafety::Normal,
+            abi: abi::Abi::Rust,
+        }
+    }
 }
 
 pub type PolyFnSig<'tcx> = Binder<FnSig<'tcx>>;
@@ -1955,6 +1965,9 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
                 tcx.fn_sig(def_id).subst(tcx, substs)
             }
             FnPtr(f) => f,
+            Error => {  // ignore errors (#54954)
+                ty::Binder::dummy(FnSig::fake())
+            }
             _ => bug!("Ty::fn_sig() called on non-fn type: {:?}", self)
         }
     }

src/test/ui/issues/issue-54954.rs

@@ -0,0 +1,19 @@
+#![feature(const_fn)]
+
+const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
+//~^ ERROR constant contains unimplemented expression type
+
+trait Tt {
+    const fn const_val<T: Sized>() -> usize {
+    //~^ ERROR trait fns cannot be declared const
+        core::mem::size_of::<T>()
+    }
+}
+
+fn f(z: [f32; ARR_LEN]) -> [f32; ARR_LEN] {
+    z
+}
+
+fn main() {
+    let _ = f([1f32; ARR_LEN]);
+}

src/test/ui/issues/issue-54954.stderr

@@ -0,0 +1,16 @@
+error[E0379]: trait fns cannot be declared const
+  --> $DIR/issue-54954.rs:7:5
+   |
+LL |     const fn const_val<T: Sized>() -> usize {
+   |     ^^^^^ trait fns cannot be const
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/issue-54954.rs:3:24
+   |
+LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0019, E0379.
+For more information about an error, try `rustc --explain E0019`.